Embodiments of the invention relate to a complementary metal oxide semiconductor (CMOS) image sensor.
Generally, the image sensor is a semiconductor device converting an optical image into an electrical signal. Such image sensors include charge coupled devices (CCD) where individual metal-oxide-silicon (MOS) capacitors are adjacent to each other and store charge carriers and transfer them, and CMOS (complementary MOS) image sensors adopting a switching manner that includes a number of MOS transistors somewhat dependent on the number of pixels and that uses a CMOS technology for making peripheral circuits, a control circuit, and a signal processing circuit that sequentially detect and output image data. The CMOS image sensor converts the optical information of a subject into electrical signals by signal processing devices including photodiodes, an amplifier, an A/D converter, an internal voltage generator, a timing generator, digital logic, etc., included in one chip, having great advantages of reduction of space, power, and costs.
Meanwhile, CMOS image sensors include a 3T type, a 4T type, a 5T type, etc., according to the number of transistors per unit pixel. The 3T type is constituted by one photodiode and three transistors per unit pixel, and the 4T type is constituted by one photodiode and four transistors per unit pixel.
Herein, the layout for the unit pixel of the 4T type CMOS image sensor will be described. FIG. 1 is an equivalent circuit view of a 4T type CMOS image sensor of the related art, and FIG. 2 is a layout showing a unit pixel of a 4T type CMOS image sensor of the related art.
As shown in FIGS. 1 and 2, a unit pixel 100 of a CMOS image sensor comprises a photodiode 10 as a photoelectric converter and four transistors. The four transistors are a transfer transistor 20, a reset transistor 30, a drive transistor 40, and a select transistor 50, respectively. And, a load transistor 60 is electrically connected to the output terminals OUT of the respective unit pixels 100.
Herein, FD represents a floating diffusion region, Tx represents the gate voltage of the select transistor 20, Rx represents the gate voltage of the reset transistor 30, Dx represents the gate voltage of the drive transistor 40 (and which is also the voltage on the floating diffusion region FD), and Sx represents the gate voltage of the select transistor 50.
In the unit pixel of the 4T type CMOS image sensor in the related art, an active region is defined so that a device isolating layer is formed in a portion of the substrate other than the active region, as shown in FIG. 2. One photodiode PD is formed in the portion of the active region having a wide width, and the gate electrodes 23, 33, 43, and 53 of the four transistors are formed in another portion of the active region. In other words, a transfer transistor 20 includes the gate electrode 23, a reset transistor 30 includes the gate electrode 33, a drive transistor 40 includes the gate electrode 43, and a select transistor 50 includes the gate electrode 53.
Herein, the active regions of the respective transistors (excluding the channel under the respective gate electrodes 23, 33, 43, and 53) are implanted with impurity ions so that source/drain (S/D) regions of the respective transistors are formed.
When the entire well-capacity of the photodiode PD is larger than the charge holding capacity of the floating diffusion area FD, the charge between the photodiode and the floating diffusion area is shared. In this case, if the gate electrode 23 of the transfer transistor returns to an off state, the photodiode still has a signal or charge, which will be mixed with the signal or charge generated in the next frame, thereby leading to image lag. This saturation of the floating diffusion node 25 usually limits the dynamic range of a 4T pixel.
Further, as the pixel becomes smaller, the capacity of the floating diffusion area becomes smaller. This makes the dynamic range of the pixel much smaller. Accordingly, even when the pixel is small, a need exists for an improvement of the dynamic range to provide a good output response for both low light and high light conditions.